X-lock is a remote lockout like pop-lock, but it's hydrolic, rather than a cable… so more reliable and nicer action. Also, it has a remote barrel adjuster (on the bar mounted button) for adjusting the low speed compression damping, which can be useful. I never liked pop lock and so took it off my old Revelations and switched to a standard leg top lever for lock out and only used it for fire roads.
Now I've got x-lock I use lock out really regularly, I can jump out the saddle to sprint up a short climb and push the x-lock button to stiffen up the front super quick… I am aware that I am starting to sound like an advert here. Anyway, I like it, but I use it a lot.